This application claims the priority benefit of Taiwan application serial no. 90127629, filed Nov. 7, 2001.
1. Field of the Invention
The present invention relates to an overlay mark and method of application thereof, and in particular, an overlay mark for concurrently monitoring the alignment accuracy, focus, leveling and astigmatism and method of application the overlay mark.
2. Description of the Related Art
As the dimension of the semiconductor becomes smaller, and with higher levels of integration, the fabrication processes are more complicated and more difficult. Thus, the direction of semiconductor manufacturers has turned to monitoring and controlling, by employing real-time measuring devices, to real-time respond/solve problems so as to lower damages caused by fabrication process errors.
Generally, other than the control of the critical dimension of a wafer, the factor governing the success or failure of a wafer photolithography process is alignment accuracy. Thus, alignment accuracy measurement, or overlay error measurement, is an important task in the semiconductor fabrication process. An overlay mark is applied as a tool for measuring overlay error and is used to determine the alignment accuracy of the pattern of a photoresist layer after a photolithography process with that of a previous layer over the chip.
FIG. 1 is an aerial view of a conventional overlay mark for monitoring alignment accuracy.
Referring to FIG. 1, an overlay mark is formed on a specific wafer, for monitoring alignment accuracy and further, the monitoring process is executed only at a specific time. The overlay mark includes four inner bars 100 and four outer bars 102, wherein the outer bars 102 represent a secured fore-layer position, and the inner bars 100 represents the pattern of the photoresist layer after the photolithography process. In other words, the outer bars 102 are used as a base for the inner bars 100. The layout is that the inner bars 100 and outer bars 102 respectively form into rectangles. Each bar is a side of the rectangle and the sides are not connected, wherein the rectangle formed by the outer bars 102 encloses the rectangle formed by the inner bars 100.
In the process of monitoring alignment accuracy, a monitoring beam scans in a scanning direction 104 across the two outer bars 102 and two inner bars 100, as shown in FIG. 1. After scanning, signals of the actual position representing respectively the outer bars 102 and the inner bars 100 are read, and, next, signals representing the mean value of the position of the two outer bars 102 are measured, compared, and the differences, i.e, overlay error, are calculated. If the overlay error is larger than the acceptable deviation value, this means that the alignment between the pattern of the photoresist layer and that of the chip has not reached the accuracy requirements, and the photoresist layer has to be removed, and a second photolithography process has to be repeated until the overlay error is smaller than the acceptable deviation value. For detailed structure of the overlay mark, refer to FIG. 1.
FIG. 2 is a sectional view taken along line Ixe2x80x94I of FIG. 1.
FIG. 2 shows a conventional structure of an overlay mark, which has a substrate 200 having a deposition layer 202 being etched to form a trench 204, and a photoresist pattern 206 on the deposition layer 202, wherein the photoresist pattern 206 is located at the inner side of the trench 204.
The relationship of FIGS. 1 and 2 shows that the trench 204 in FIG. 2 corresponds to the outer bars 102 and the photoresist pattern 206 shown in FIG. 2 corresponds to the inner bars 100 of FIG. 1.
In the exposure process, normally the pattern shown in FIG. 1 is formed over a specific wafer and is applied as an overlay mark for the monitoring process of alignment accuracy, and the monitoring process is only executed at a specific time. Although alignment accuracy can be monitored, the time used in fabrication process is increased, and real-time monitoring of alignment accuracy is not possible. Besides, after photolithography the pattern needs to undergo multiple monitoring processes to ensure throughput of the product and to avoid the lowering of yield. The to-be-monitored items of the monitoring process include focus of the pattern, leveling and astigmatism. In order to monitor focus, leveling and astigmatism, these items have to be tested individually and therefore the production time for mass production suffers and real-time monitoring cannot be achieved.
Accordingly, it is an object of the present invention to provide an overlay mark for concurrently monitoring alignment accuracy, focus, leveling and astigmatism and a method of application thereof, wherein the throughput of production is increased and the yield of the product is improved.
Yet another object of the present invention is to provide an overlay mark for concurrently monitoring alignment accuracy, focus, leveling and astigmatism and method of application thereof. The overlay mark comprises four inner bars and four outer bars, wherein each inner bar has a sawtooth area and a bar-shaped area, and the outer bar is a fore-layer etched pattern. The layout is that the four inner bars form into a rectangle and each inner bar is one side of the rectangle. None of the sides are connected. The sawtooth areas of the inner bars disposed on opposite sides are located at a same position. The four outer bars also form into a rectangle and each outer bar is a side of the rectangle. The rectangle formed by the outer bars encloses the rectangle formed by the inner bars.
In the monitoring process, a testing beam scans in a scanning direction over a scan area. The scan area is divided into two areas. The first scan area includes the sawtooth area of the opposite two inner bars and the opposite side of the two outer bars for monitoring the focus, leveling and astigmatism of the exposure areas. The second scan area includes the opposite sides of the two outer bars and the bar-shaped area of the opposite sides of the two inner bars for monitoring alignment accuracy of the exposure areas.
The principle of monitoring of the present invention is by employing line-end shortening of inner bars having sawtooth area formed during defocus and employing the characteristic of non-influence by the defocus at the etched fore-layer of the outer bars. Thus, the inner bars of the sawtooth area cause a center shift as a result of the defocus during the measuring of alignment accuracy, and thus, by reverse calculation of the amount of center shift, a relative defocus is obtained. Based on this principle, the leveling and astigmatism can be obtained.
In accordance with the present invention, the method of monitoring is that
First, the measured center position of the outer bar is used as base, and the measured center position of the bar-shaped area of the inner bar is compared with that of the outer bar. Calculate the differences, and the alignment accuracy can then be calculated.
Next, take the earlier measured alignment accuracy of the second scan area as a reference set, i.e., the error caused by alignment accuracy is first excluded. Next, note the measured offset of the x-axis alignment accuracy of the sawtooth area. The focus can be then be computed.
Further, by measuring the offset of x-axis and y-axis alignment accuracy of the sawtooth area of the first scan region, the astigmatism can be calculated.
If leveling is to be measured, the offsets of the x-axis alignment accuracy of the sawtooth area of the inner bar of the overlay mark at the individual corners on the exposure areas are measured and then compared, then, the leveling can be computed.
Accordingly, the present invention, unlike the conventional method of forming an overlay mark on a specific wafer and executing the monitoring process at a specific time, instead forms an overlay mark over the corners of the to-be-tested exposure areas of each wafer. Thus, the present method provides real time and concurrent monitoring of alignment accuracy, focus, leveling and astigmatism so as to save fabrication time and avoid throughput from suffering. In addition, the present invention is employed directly over the product and the products can be optionally inspected, and, therefore, the production yield is greatly improved.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.